A Sterically Encumbered Triarylborane Enabling CO2 Reduction to Formate by Electrons and Protons

Abstract Borohydrides are widely used sources of hydride ions (H–), but their conventional synthesis from highly reducing metal hydrides is energetically demanding and undermines sustainable applications. Herein, we report a sterically encumbered thioether-substituted triaryl borane, 1,2-(tBuS)C6H4(BMes2) (3) (Mes = 2,4,6-Me3C6H2), which is highly air- and moisture-stable. 3 exhibits a one-electron reduction at E0 = −2.62 V vs Fc+|0 and a radical anion half-lifetime of ∼0.5 s in THF, enabling bimolecular follow-up reactivity. Indeed, two-electron reduction and protonation of 3 with KC8/[DBUH]+ (DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene) or hydride transfer from Li(AlH4) affords the borohydride 4–. From a hydricity–potential correlation, we estimate a hydricity of ∼36 kcal mol–1 in MeCN, which allows 4– to reduce H+ to H2 and CO2 to formate under ambient conditions in almost quantitative yields. The reaction of 3, [DBUH]+, KC8, and CO2 furnishes potassium formate in ∼90% spectroscopic yield, establishing a closed synthetic cycle for CO2 reduction by protons and electrons at a boron center. These results highlight how tailor-made, redox-active boranes can complement transition-metal hydrides as modular, metal-free platforms for coupling renewable electrons and protons in CO2 utilization chemistry.